The present invention relates to a color cathode ray tube; and, more specifically, the invention relates to a color cathode ray tube in which beam landing errors resulting from the movement of a shadow mask structure due to a temperature rise hardly occur.
A color cathode ray tube generally comprises a front panel having a phosphor screen that serves as a picture screen, a neck portion accommodating the electron guns, and a funnel portion connecting the front panel and the neck portion. Electron beams emitted from the electron guns are deflected in horizontal and vertical directions by a deflection yoke on their way to the phosphor screen so that they are scanned two-dimensionally over the phosphor screen formed on the inner surface of the panel to form an image on the screen. At this time, the electron beams for red (R), green (G) and blue (B) phosphors are selected by a shadow mask installed on the inner side of the panel and impinge on the phosphors of corresponding colors, causing these phosphors to emit light and form an image on the phosphor screen.
The shadow mask structure comprises a shadow mask having a large number of electron beam passing openings, a support frame for holding the shadow mask, and mask springs for holding the support frame inside the panel of the color cathode ray tube. The shadow mask structure is suspended inside the panel and is supported through the mask springs on panel pins embedded in the panel. The shadow mask is made of, for example, invar (with an expansion coefficient of, for instance, 1.5.times.10.sup.-6 /.degree. C.), the support frame is made of steel (with an expansion coefficient of, say, 1.09.times.10.sup.-5 /.degree. C.), and the mask spring is made of stainless steel (with an expansion coefficient of, say, 1.04.times.10.sup.-5 /.degree. C.). The expansion coefficient referred to here is the coefficient of linear expansion.
FIG. 1 shows the relation between the ambient temperature and the beam drift. The panel is normally formed of glass. The expansion coefficient of glass is 8.times.10.sup.-6 to 10.times.10.sup.-6 which is larger than that of a shadow mask assembly having the above construction. Reference number la represents the glass panel at an ambient temperature of 20.degree. C. and 1b represents the glass panel at 40.degree. C. It is seen that the panel 1b is expanded circumferentially outwardly compared with the panel 1a. The shadow mask 2 on the other hand remains in almost the same state with a change in the ambient temperature. Thus, a phosphor screen portion 3, on which the electron beam B after passing through an opening in the shadow mask strikes when the panel is in the state 1a, moves circumferentially outwardly to a position 4 when the panel is in the state 1b. That is, the electron beam apparently shifts toward the center of the panel in that the electron bream will now strike a point closer to the center of the panel 1b relevant to the position 4.
Such a drift of the electron beam on the phosphor screen depending on the ambient temperature surrounding the cathode ray tube, which is generally called an electron beam drift, is hereinafter referred to as an ambient temperature drift. The ambient temperature drift causes a color purity defect. The ambient temperature drift apparently causes the electron beam to move inwardly over the phosphor screen, as discussed above. The mask springs are made of a bimetal to improve the ambient temperature drift. The warp of the bimetal mask springs is utilized to move the whole shadow mask away from the phosphor screen to optimize the displacement of the electron beam and therefore prevent a degradation of the color purity. When different materials are used for the shadow mask and the support frame, the amounts of their expansion differ because they have different expansion coefficients.
When such a cathode ray tube is operated, a generally-called doming phenomenon occurs in which the shadow mask starts to expand immediately after the start of tube operation and moves toward the phosphor screen. The doming phenomenon in which the shadow mask moves toward the phosphor screen can be suppressed because the invar material used for the shadow mask has a relatively small expansion coefficient. When the display tube continues to be used for a long period of time, the shadow mask is extended circumferentially outwardly by the expansion of the support frame because the amounts of expansion of the shadow mask and the support frame differ. This results in generally-called electron beam drift in which the electron beam is displaced over the phosphor screen. The electron beam drift that results from the difference in expansion between the shadow mask and the support frame when the tube is used for a long period will be hereinafter referred to as a long time drift. The long time drift causes the electron beam to move circumferentially outward over the phosphor screen, resulting in a color purity defect.
FIG. 2 shows the amount of beam displacement when bimetal mask springs are used. The beam displacement is given a plus sign (+) when the electron beam, after having passed through the electron beam passing opening, apparently moves toward the center of the panel when viewed from the front of the panel and a minus sign (-) when it apparently moves outwardly. In a cathode ray tube which has been optimally set at the ambient temperature of 20.degree. C. when manufactured, for example, the electron beam displacement when the tube is used at the ambient temperature of 20.degree. C. is represented by line 5 and the electron beam displacement at 40.degree. C. is represented by line 6. When bimetal mask springs are used, the distance between lines 5 and 6 is narrow, and the ambient temperature drift is improved, whereas, as for the long time drift, the shadow mask moves further away from the phosphor screen, increasing the drift of the electron beam.
FIG. 3 shows the amount of beam displacement when monometal mask springs are used. The beam displacement is given a plus (+) sign when the electron beam, when viewed from the front of the panel, apparently moves toward the center of the panel and a minus (-) sign when it apparently moves outward. The use of monometal mask springs can suppress the displacement of the electron beam by the offset produced by the ambient temperature drift in view of the long time drift. The electron beam displacement, however, is greatly affected by the ambient temperature at which the tube is used, resulting in a wide range of variation of the displacement. In a cathode ray tube which has been optimally set at the ambient temperature of 20.degree. C. when manufactured, for example, the electron beam displacement when the tube is used at the ambient temperature of 20.degree. C. is represented by line 7, and the displacement when the tube is used at 40.degree. C. is represented by line 8. In this case, there is an ambient temperature difference of 20.degree. C. The maximum difference of the electron beam displacement at this time is therefore greatly affected by the ambient temperature. Further, the color purity setting is already exceeded before operating the tube.
Although the use of the monometal mask springs can suppress the electron beam displacement due to the long time drift by the offset produced by the ambient temperature drift, the use of monometal mask springs leads to a bad characteristic of the ambient temperature drift. It has therefore been difficult to improve both electron beam drifts, i.e., the ambient temperature drift and the long time drift, at the same time, in that either the ambient temperature drift or the long time drift must be sacrificed, even with the use of bimetal or monometal mask springs for the sides of such a conventional shadow mask structure.
A tube with a dot type phosphor screen structure, in particular, has a purity problem significantly more serious than that of the stripe type phosphor screen structure. Further, in high definition color display tubes with a shadow mask hole pitch, which determines the phosphor screen dot pitch, of below 0.31 mm, the problem is more serious. This purity problem with high definition displays with substantially more than 1,000 horizontal scanning lines is also critical.
To solve these problems, various techniques have been disclosed in Japanese Patent Laid-Open Nos. 14851/1989, 209635/1989 and 44915/1994, but they can solve only one of the problems of ambient temperature drift and long time drift.